Digitizers, as known in the art, are very useful for translating the position of an item in a plan or drawing into coordinates recognizable by a computer. In the latter form, the computer can make use of the position information in any number of useful ways. Unlike the position information from a mouse, which is always relative to its immediately previous position on a support surface, the digitizer position information is relative to a grid of conductors incorporated into its work surface. It becomes, therefore, absolute with respect to the work surface and any plan or drawing mounted thereon in a designated orientation. As a result, the position information is accurate enough not only to edit graphic displays, but also to control manufacturing processes according to scale drawings placed on the work surface, and to control navigation of air and water craft according to charts.
A number of well known digitizer systems make use of a movable coil and a work surface defined by a grid of conductors. The coil may be disposed within the tip of a pen-shaped instrument so that the pen point accurately locates the effective coil center. Alternatively, the coil may surround a transparent disc with a set of cross hairs etched thereon to mark the coil center in what is known as a cursor. The grid normally comprises a set of parallel conductors spaced along the work surface in what may be called the x direction and another set similarly disposed along the work surface in the orthogonal y direction. An oscillator applies an ac signal of predetermined frequency and amplitude to the coil, which is inductively coupled to the conductors of the grid.
In accordance with the principles of well known electromagnetic theory, ac electrical signals are induced in the grid conductors at a magnitude and phase that depend on the location of the coil relative to the conductors. Generally, the signals induced in the conductors will have a magnitude that varies from zero at the coil center to a maximum at the coil periphery and tapering off beyond. Further, the phase of the signals induced in conductors at one side of the coil will be the opposite of (180 degrees displaced from) that of signals induced in conductors at the other side. Both of these electromagnetic properties are used to advantage in several known digitizers.
In these known systems, individual conductors are selectively connected, one at a time, through the use of multiplexer circuitry, to detection circuitry that determines both the phase and the magnitude of the signal induced in the selected conductor. When, for example, a conductor of the x set is selected by the multiplexer circuitry, the phase of the induced signal indicates whether the pointer is to the right or to the left of the selected conductor. The x grid may therefore be scanned by selecting conductors in succession from one end of work surface, until the phase of the induced signal reverses. The phase reversal identifies the two adjacent conductors between which the pointer is located. The precise position of the pointer between the identified adjacent conductors may then be determined by the relative magnitudes of induced signal in the identified conductors.
In the digitizer described in U.S. Pat. No. 4,734,546, which is incorporated herein by reference, and which issued on Mar. 29, 1988 to Waldo L. Landmeier, the current inventor, each conductor is looped through several areas of the work surface to reduce the number of conductors in each grid set. Looping, of course, generates segments of each conductor in which the direction across the work surface of a continuous conductor current is reversed. These segment direction reversals are observed by the detecting circuitry as phase reversals of the induced signal. Selective conductors are looped in different manners so that, in each work surface area, the combination of conductor segment directions is unique. Since the phase of the signals induced in all conductor segments to one side of the pointer is the same, the pattern of observed phase reversals among the combination of conductors is used to uniquely identify the particular area of the work surface on which the pointer coil is located. Phase reversals due to segment loop direction are then compensated for, and pointer location within the area proceeds normally.
Since the number of individual conductors to be scanned is greatly reduced by the Landmeier looping grid system, so too is the number of multiplexers required for each grid, and the attendant complexity of the pointer locating circuitry. This represents a substantial cost saving as well as a reduction in coil locating time. Obviously, however, the pointer locating circuitry does require a constant phase reference relative to the sending coil signal to unambiguously define the specific loop area in which the coil is located. This presents no problem in a digitizer system with a tethered pointer, whether the oscillator is located in the pointer or the base unit, for a phase reference signal can be carried in the tether cable. Since a tethered pointer tends to impede the pointer's movements, however, many users prefer an untethered pointer.
In U.S. Pat. No. 5,045,645 by Jason J. Hoendervoogt and James S. Watson, entitled "Digitizer System With Passive Pointer", and assigned to the assignee hereof, the required phase reference information is supplied electrostatically by capacitive coupling between the pointer and the base electronics. While this is a very effective and desirable solution, it does, nevertheless, impede if not preclude the use of electrostatic force to hold the work to the work surface. Electrostatic holding techniques, on the other hand, are very effective, inexpensive and commonly used in the digitizer industry.
An object of the present invention is to provide a digitizer system having an untethered pointer that supplies full phase information without relying on electrostatic coupling of a phase reference signal between pointer and base electronics.
Another object is to provide a cordless digitizer system utilizing electromagnetic locating signals.
Another object is a digitizer system having an untethered pointer, looping grids and electrostatic work hold-down.